XQL

XQL is a name used for several experimental XML query languages proposed in the late 1990s and early 2000s to query and retrieve data from XML documents. Because there was no single standard for XQL, different projects implemented different syntax and semantics under the XQL label. The common idea was to specify queries that traverse XML trees and extract data by matching patterns against element and attribute structures.

Designs typically used path-like expressions or pattern matching on the hierarchical structure of XML data, sometimes combining structural criteria with value constraints. Some proposals emphasized concise notation for selecting nodes, while others explored joins across multiple documents or complex transforms.

XQL contributed to the development of later XML query languages. It is distinct from XML-QL and from the W3C standards XPath and XQuery; however, elements of its approach—such as tree navigation and pattern matching—appeared in later work. In the 2000s, the W3C formed working groups that produced formal standards; XPath emerged earlier, and XQuery 1.0 became a W3C Recommendation in 2007.

Because XQL referred to several incompatible initiatives, there is no single canonical syntax. Today, XQL is largely of historical interest, cited as part of the evolution of XML querying rather than as an adopted standard. Some research prototypes and educational materials still discuss XQL as a concept or family of languages.

See also: XPath, XQuery, XML-QL, XML databases.